Angular rate sensors are designed for measuring rotational velocity around the sensitive axis of the sensor. Such a device is typically composed of a rapidly spinning wheel or disk mounted on a shaft with various kinds of supports to allow it to spin freely. Its angular momentum resists the angular motion of its base about the two axes orthogonal to its spin axis. Angular rate sensors are used in conjunction with accelerometers to make a dynamic measurement of an orientation angle; or used by themselves to track rotational motion or a rate of acceleration of a moving object, with which the sensor is associated, by sensing an effect of this movement onto an internal proof mass. Typically, such an inertial proof mass is suspended from the sensor housing or body which is rigidly mounted to the object.
Most of micro scale mechanical rate sensors, although varying in type and design, operate on similar principles: the sensor structure, when driven in a specific direction and subjected to an angular acceleration about the sensor's input axis, deflects in the direction orthogonal to the input axis due to presence of the Coriolis force. This deflection can be sensed electrostatically, magnetically, optically or using piezoresistive or piezoelectric elements. Since the Coriolis force is small due to small mass of micro scale devices, the dynamic amplification is used and the devices are typically driven at resonance. In order to exploit the dynamic amplification, resonant frequencies of the driving and sensing mode should be perfectly matched. Some examples of the devices of the kind specified are disclosed in the following patent publications: U.S. Pat. No. 5,992,233; U.S. Pat. No. 6,928,873; U.S. Pat. No. 6,792,792; U.S. Pat. No. 6,595,054. The main difficulty in this kind of devices is related to the fact that the frequencies cannot be perfectly matched due to low relative tolerances of microfabrication.
Another mechanical rate sensor is a shell gyro (e.g. U.S. Pat. No. 4,644,793). A shell gyro utilizes a mechanical scheme with degenerated, automatically matched modes. However, a shell gyro is difficult to fabricate using micromachining methods. This kind of structure implemented at micro scale is ring gyro is described for example in GB 2 276 976; U.S. Pat. No. 6,670,212; and F. Ayazi and K. Najafi, “A HARPSS Polysilicon Vibrating Ring Gyroscope”, JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 10, NO. 2, JUNE 2001.
The ring gyro vibrates in a radial direction in such a way that the shape of its vibrational mode is elliptical. When an angular rate is present in the direction perpendicular to the ring plane, the direction of the largest semi axis of the ellipse changes proportionally to the angular rate. Since the ring stiffness is identical in any direction, the frequencies matching is not necessary. However, the performance, and mainly resolution of the ring gyro, is limited by a small mass of the ring and necessity to detect very small deflections. In addition, the symmetry of the ring stiffness is typically not ideal due to suspension springs attached to it. An alternative is a magnetically or electrostatically levitated spinning disk gyro, described for example in U.S. Pat. No. 5,353,656; and C. Shearwood, K. Y. Ho, C. B. Williams, H. Gong, “Development of a levitated micromotor for application as a gyroscope, Sensors and Actuators”, 83, 2000 pp. 85-92. The spinning disks gyros potentially could demonstrate superior performance due to larger proof mass. However, the control of the disk motion is usually very complex due to an intrinsic instability of a spinning disk while the fabrication is usually challenging or requires assembly steps. In addition, in the case of magnetic levitation, levitation and control coils exhibit relatively high power consumption.
Yet another known design of an angular rate sensor is a clover leaf gyroscope. This is described for example in U.S. Pat. No. 5,894,090. Such a device also presents a vibrational type sensor.